Re: vsta on biprocessors pentium

 Andy writes:
> [basile@soleil.serma.cea.fr (Basile STARYNKEVITCH) writes:]
>
> >Some manufacturers are making biprocessors pentium 90 motherboards.
> >I'm considering buying such a thing.
> >Does it make sense?
> >Are such motherboards useable?
>
> I'm told (by friends in the PC system business) that the current breed of
> dual-Pentium motherboards are pretty bad. Some of them which have come out
> in the last month or two are better.

One big problem is that that the current low-end crop share the L2 cache,
so the amount of parallelism you get is limited to the extent that the
pentiums run out of their internal cache. I've heard numbers like 15%-20%.

> >Can these motherboards run (in monoprocessor mode) popular OSes such
> >as Linux?
>
> I understand that one of the CPUs remains offline until you take a
> particular action.

Yes, definitely - the second processor has to be explicitly activated.

> >Does anyone have hints, feature to check (eg cache consistency between
> >both processors), motherboard brands to avoid, etc?
>
> You want a coherent physical cache if at all possible. Bus snooping is
> higly desirable, otherwise you have to be shooting down cache lines as
> you do DMA.

The Intel MP Spec, which these boards adhere to, requires that the hardware
maintain cache coherency, transparent to the OS. The OS does have to do
TLB shootdowns.

You can get the Intel MP spec in postscript from the Intel on-line document
repository at www.intel.com. You do have to fill out a 'registration' form
to get access to the documents. The intel pentium book also has a chapter
detailing the on-chip APIC and multiprocessing support.

I just bought an ASUS P54NP4 dual processor board, but I haven't started MP
hacking on it yet. At least the first processor runs well. :-)

With just 15-20% performance improvement, for double the cost, I wouldn't
recommend the shared L2 cache dual pentiums to anyone, UNLESS you
specifically want to hack on SMP support on the cheap, like me.

Jaime
...............................................................................
: James da Silva : UCMP Computer Science Dept. : Stand on my shoulders, :
: jds_at_cs.umd.edu : http://www.cs.umd.edu/~jds : not on my toes. :
Received on Fri Feb 17 07:59:08 1995